Optical pK_a Control in a Bifunctional Iridium Complex

Abstract

There are few ways to switch a catalyst's reactivity on or off, or change its selectivity, with external radiation; many of these involve photochemical activation of a catalyst. In the case of homogeneous late-transition-metal catalysts, the metal complex itself is frequently the chromophore involved in such reactivity switching. We show here a base-pendant ligand−metal bifunctional scaffold wherein a photobase, a compound that becomes more basic in the excited state (pK a < pK a *), is used to switch the proton acceptor ability on an active site of the complex. The system differs from those with metal-centered chromophores, because the photobase operates independently of the metal. While excellent progress has been made in photoacid chemistry, neither a photoacid nor a photobase has been designed into the structure of a transition-metal catalyst where the metal is not part of the chromophore. We find that quinoline is an efficient photobase that preserves its unique properties in the close proximity of an iridium center: the efficacy of the photobase (9.3 < pK a * < 12.4) in the iridium complex is unhindered relative to the free quinoline. We apply this notion to successful photodriven deprotonation of an aliphatic alcohol, thus showing the first case of metal-orthogonal optical pK a control in a transition-metal complex.

Structure–Photochemical Function Relationships in Nitrogen-Containing Heterocyclic Aromatic Photobases Derived from Quinoline

“…[12][13][14][15][16][17][18][19] Recent experimental studies performed in the Dawlaty lab have investigated the photochemical properties of a family of 5-R quinoline derivatives. [20][21][22][23][24][25] These compounds were all shown to be photobases, with the magnitude of the photobasicity depending strongly on the identity of the substituent. Specifically, photoexcitation results in Kb increasing by over 10 orders of magnitude when the substituent is the electron-donating NH2 group but only approximately 2 orders of magnitude when the substituent is the electron-withdrawing CN group.…”

Structure–Photochemical Function Relationships in the Photobasicity of Aromatic Heterocycles Containing Multiple Ring Nitrogen Atoms

“…26 Moreover, two iridium complexes containing pendant quinoline ligands were reported in which the quinoline moiety retained its photobasicity. 27 This represents the first example of a rationally designed transition-metal complex that exhibits photoactivated proton removal functionality.…”

“…A series of recent experimental studies by Dawlaty and co-workers focused on the photochemical properties of a family of photobases derived from quinoline. − In a series of 5- R -quinolines, they demonstrated that the magnitude of the photobasicity depends strongly on the identity of the substituent, with electron-donating substituents facilitating stronger photobasicity than electron-withdrawing groups. In particular, photoexcitation results in the K b of 5-aminoquinoline increasing by over 10 orders of magnitude whereas the K b of 5-cyanoquinoline only increases by approximately 2 orders of magnitude .…”

“…Further experimental studies focused on determining the role of triplet states in the photochemistry of the 5- R -quinolines, the importance of solvent-mediated stabilization of the anion generated through the excited-state proton transfer, and the tunability of the energetic cost of photobasicity . Moreover, two iridium complexes containing pendant quinoline ligands were reported in which the quinoline moiety retained its photobasicity . This represents the first example of a rationally designed transition-metal complex that exhibits photoactivated proton removal functionality.…”

Structure-Photochemical Function Relationships in Nitrogen-Containing Heterocyclic Aromatic Photobases Derived from Quinoline

“…[12][13][14][15][16][17][18][19] Recent experimental studies performed in the Dawlaty lab have investigated the photochemical properties of a family of 5-R quinoline derivatives. [20][21][22][23][24][25] These compounds were all shown to be photobases, with the magnitude of the photobasicity depending strongly on the identity of the substituent. Specifically, photoexcitation results in Kb increasing by over 10 orders of magnitude when the substituent is the electron-donating NH2 group but only approximately 2 orders of magnitude when the substituent is the electron-withdrawing CN group.…”